Underground mineral oil formations typically have relatively high temperatures. After the production of the crude oil to the surface, the crude oil produced therefore cools down to a greater or lesser degree according to the production temperature and the storage or transport conditions.
According to their origin, crude oils have different proportions of waxes, which consist essentially of long-chain n-paraffins. According to the type of crude oil, the proportion of such paraffins may typically be 1 to 30% by weight of the crude oil. When the temperature goes below a particular level in the course of cooling, the paraffins can crystallize, typically in the form of platelets. The precipitated paraffins considerably impair the flowability of the oil. The platelet-shaped n-paraffin crystals can form a kind of house-of-cards structure which encloses the crude oil, such that the crude oil ceases to flow, even though the predominant portion is still liquid. The lowest temperature at which a sample of an oil still just flows in the course of cooling is referred to as the pour point (“yield point”). For the measurement of the pour point, standardized test methods are used. Precipitated paraffins can block filters, pumps, pipelines and other installations or be deposited in tanks, thus entailing a high level of cleaning.
The deposit temperature of oil deposits is generally above room temperature, for example 40° C. to 100° C. Crude oil is produced from such deposits while still warm, and it naturally cools more or less quickly to room temperature in the course of or after production, or else to lower temperatures under corresponding climatic conditions. Crude oils may have pour points above room temperature, such that crude oils of this kind may solidify in the course of or after production.
It is known that the pour point of crude oils can be lowered by suitable additives. This can prevent paraffins from precipitating in the course of cooling of produced crude oil. Suitable additives firstly prevent the formation of said house-of-cards-like structures and thus lower the temperature at which the crude oil solidifies. In addition, additives can promote the formation of fine, well-crystallized, non-agglomerating paraffin crystals, such that undisrupted oil transport is ensured. Such additives are also referred to as pour point depressants or flow improvers.
Paraffin inhibitors or wax inhibitors refer to those substances intended to prevent the deposition of paraffins or paraffin waxes on surfaces in contact with crude oils or other wax-containing oils and/or mineral oil products.
The use of ethylene copolymers as flow improvers is known, especially that of copolymers of ethylene and unsaturated esters. Examples thereof are described in DE-A-21 02 469 or EP 84 148 A2.
DE-A-16 45 785 discloses heating oil mixtures with a depressed pour point. The mixtures comprise at least 3% by weight of polymers having unbranched saturated side chains having at least 18 carbon atoms, for example homo- or copolymers of alkyl esters of unsaturated mono- and dicarboxylic acids and homo- or copolymers of various alkyl vinyl ethers.
DE-A-20 47 448 discloses additives for lowering viscosity in paraffin-based crude oils. The additives are mixtures of polyvinyl ethers and ethylene-vinyl acetate copolymers.
EP 486 836 A1 discloses mineral oil middle distillates, for example gas oils, diesel oils or heating oil, which comprise polymeric additives to improve the flow properties under cold conditions. The polymeric additives are a combination of customary ethylene-based flow improvers, for example copolymers of ethylene and vinyl acetate, vinyl propionate or ethylhexyl acrylate and copolymers of linear or branched C8- to C18-alkyl (meth)acrylates and/or linear or branched C18- to C28-alkyl vinyl ethers in a weight ratio of 40:60 to 95:5, and the copolymers of the alkyl (meth)acrylates and/or alkyl vinyl ethers and the conventional flow improvers may be in the form of a mixture or the copolymers of the alkyl (meth)acrylates and/or alkyl vinyl ethers may wholly or partly be grafted onto the conventional flow improvers. The alkyl radicals are preferably unbranched, but up to 20% by weight of cyclic and/or branched moieties may be present. In the sole example for preparation of a graft copolymer, n-dodecyl acrylate and n-octadecyl vinyl ether are grafted onto a copolymer of ethylene and vinyl propionate having a mean molar mass Mn of approx. 2500 g/mol. The solvent used for the preparation is isoundecane, and aromatic solvents are used at a later stage for dilution.
U.S. Pat. No. 4,608,411 discloses graft copolymers for prevention of wax deposition from crude oils. The main chain consists of a copolymer of ethylene and a monomer selected from the group of vinyl esters of C2- to C18-monocarboxylic acids, C1- to C12-esters of unsaturated monocarboxylic acids or unsaturated α,β-dicarboxylic acids, or the esters or anhydrides thereof. Onto this are grafted homo- or copolymers of alkyl acrylates, the alkyl group thereof having at least 12 carbon atoms and at least 20% of the alkyl groups having at least 22 carbon atoms. The solvents proposed are various hydrocarbons.
Such graft copolymers for use as pour point depressants are typically prepared in chemical production sites, and the products are transported from there to the site of use, for example to an oilfield or to an offshore platform. Such sites of use may be in cold regions of the Earth. In order to save transport costs, concentrates of the graft copolymers in hydrocarbons are typically produced, for example concentrates having a polymer content of 50 to 80% by weight of polymers. Such concentrates can be used as such or can be formulated by users on site in the desired manner to give ready-to-use formulations. For example, dilution with solvent and/or addition of further additives is possible.
Particularly advantageous pour point depressants can be obtained by preparing said graft copolymers based on ethylene-vinyl ester copolymers using alkyl (meth)acrylates having C18 to C22 carbon radicals. Such products, however, have the disadvantage that the solutions thereof in hydrocarbons, especially the concentrates mentioned, can solidify in the course of cooling to room temperature. They accordingly first have to be melted for use, which means additional work for the user.